U.S. Patent application Ser. No. 07/727,806, filed Jul. 8, 1992 now. U.S. Pat. No. 5,221,369 discloses a process for producing in-situ heat treating atmospheres from non-cryogenically generated nitrogen. According to the application, suitable atmospheres are produced by 1) mixing noncryogenically generated nitrogen containing up to 5 vol. % residual oxygen with a hydrocarbon gas, 2) feeding the gaseous mixture into the heating zone of a furnace through a diffuser, and 3) converting the residual oxygen to an acceptable form such as a mixture of moisture and carbon dioxide, a mixture of moisture, hydrogen, carbon monoxide, and carbon dioxide, or a mixture of carbon monoxide, moisture, and hydrogen. The flow rate of a hydrocarbon gas is controlled in such way that it is always greater than the stoichiometric amount required for the complete conversion of residual oxygen to a mixture of moisture and carbon dioxide.
According to the application, acceptable atmospheres for oxide and bright annealing carbon steels are produced by carefully selecting the furnace temperature and amounts of residual oxygen and hydrocarbon gas employed. The operating regions for oxide and bright annealing claimed in this patent application are noted to be narrow. For example, an atmosphere acceptable for bright annealing carbon steels is produced at 950.degree. C. with the use of natural gas when the amount of residual oxygen present in the feed gas is limited to 0.1 vol. % or less. It is therefore desirable to 1) develop processes for using non-cryogenically generated nitrogen containing more than 0.1% residual oxygen at 950.degree. C. for producing atmosphere suitable for bright annealing carbon steels and 2) expand operating regions for oxide and bright annealing carbon steels.